Silver halide is naturally sensitive to only limited portions of the electromagnetic spectrum and its sensitivity within the limited range is low. It is conventional in the photographic art to broaden the range of sensitivity by spectral sensitization of the silver halide grains using sensitizing dyes. It is also conventional to increase the sensitivity of the grains themselves by treating them chemically during growth or ripening or after formation. Chemical sensitization is traditionally performed with sulfur sensitizers (particularly thiosulfate) and gold compounds.
The compounds used in chemically sensitizing silver halide or their by-products remain in the silver halide emulsion or on the silver halide grains after chemical sensitization has been completed. This along with other materials and physical conditions allows additional changes in sensitivity to occur after formulation of the final silver halide emulsion. Although these changes may include an increase of speed on aging, such changes are undesirable. Users of photographic materials must be assured of photographic properties and particularly the speed and contrast of the material in order to properly use the photographic element. Uncontrolled increases in speed would lead to overexposure of film by users if subtle alterations in exposure were not made by the photographer. It would be far better if the speed of photographic films could be stabilized against changes with aging.